The domestically developed HSR-350x, which achieved 352.4 km/h (219.0 mph) in tests, resulted in a second type of high-speed trains now operated by Korail, the KTX-Sancheon, which entered into commercial service in 2010.

[6] During the following years, several feasibility studies were prepared for a high-speed line with a Seoul–Busan travel time of 1 hour 30 minutes, which gave positive results.

[9] As planning progressed, the Korea High Speed Rail Construction Authority (KHSRCA) was established in March 1992 as a separate body with its own budget responsible for the project.

[9] However, plans were changed repeatedly,[14] in particular those for city sections, following disputes with local governments,[15] while construction work suffered from early quality problems.

[25] The infrastructure and rolling stock were created in the framework of a technology transfer agreement, which paired up Korean companies with core system supplier Alstom and its European subcontractors for different subsystems.

[31] After 12 years of construction and with a final cost of ₩12,737.7 billion,[32] the initial KTX system with the first phase of the Gyeongbu HSR went into service on April 1, 2004.

[34] In August 2006, the project was modified to again include the Daejeon and Daegu urban area passages, as well as additional stations along the phase 1 section.

[38] A long dispute concerning the environmental impact assessment of the Wonhyo Tunnel, which passes under a wetland area,[37] caused delays for the entire project.

[39] The dispute gained nationwide and international attention due to the repeated hunger strikes of a Buddhist nun, led to a suspension of works in 2005,[40] and only ended with a supreme court ruling in June 2006.

[32] The last element of the original project that was shelved in 1998, separate underground tracks across the Seoul metropolitan area, was re-launched in June 2008, when an initial plan with a 28.6 km (17.8 mi) long alignment and two new stations was announced.

[48] The Ulsan–Gyeongju–Pohang section of the Donghae Line is foreseen for an upgrade in a completely new alignment that circumvents downtown Gyeongju and connects to the Gyeongbu High Speed Railway at Singyeongju Station, allowing for direct KTX access to the two cities.

[49] On September 1, 2010, the South Korean government announced a strategic plan to reduce travel times from Seoul to 95% of the country to under 2 hours by 2020.

[50] The main new element of the plan is to aim for top speeds of 230–250 km/h (143–155 mph) in upgrades of much of the mainline network with view to the introduction of KTX services.

[50] In conjunction with the award of the 2018 Winter Olympics to PyeongChang in July 2011, KTX service via the eastern coast line was anticipated; the expected travel time there from Seoul is 50 minutes.

[60] As the proposal was popular with lawmakers from South Jeolla Province, the government is conducting a feasibility study, but the Jeju governor expressed skepticism.

[89] KTX-Eum is South Korea's first high-speed electric multiple unit train, being a commercialised version of the experimental HEMU-430X previously tested by Korail.

[98] Following a phase of test operation, regular KTX service started on April 1, 2004, with a maximum speed of 300 km/h (186 mph) achieved along the finished sections of the Gyeongbu HSR.

From June 2007 until October 2010, some trains left the Gyeongbu HSR between Daejeon and Dongdaegu to serve Gimcheon and Gumi before the opening of an extra station for the two cities on the high-speed line.

[108] On December 15, 2010, the new Gyeongjeon KTX service started[47] with a minimum travel time of 2 hours 54 minutes[109] over the 401.4 km (249.4 mi) long route between Seoul and Masan.

[116] The fare system implemented at the start of service in April 2004 deviated from prices proportional with distance, to favour long-distance trips.

[118] From November 1, 2006, due to rising energy prices,[121] Korail applied an 8-10% fare hike for various train services, including 9.5% for KTX.

[44] In forecasts prepared after the decision to split the project into two phases, the expected first year ridership of Gyeongbu KTX services was reduced by about 40%.

[130] On January 14, 2005, Prime Minister Lee Hae Chan stated that "the launch of KTX was a classic policy failure" due to construction costs significantly above and passenger numbers well below forecasts.

[147] Lawmakers criticised the safety of Korail's tunnels after the Ministry of Construction and Transportation submitted data to the National Assembly on June 13, 2005.

[149] On October 5, 2008, it was revealed by lawmakers that inside Hwanghak Tunnel, from December 2004, inspectors have monitored the progression of several cracks and minor track displacements, which continued after maintenance work in March–April 2007 and again in March 2008.

[150] The operator claimed that a February 2007 on-site inspection found the problems not safety-relevant, but pledged further maintenance, and an investigation into the causes was launched.

[162] On February 11, 2011, a KTX-Sancheon train[165] bound for Seoul from Busan derailed on a switch in a tunnel 500 m (1,600 ft) before Gwangmyeong Station,[166] when travelling at around 90 km/h (56 mph).

[168] Investigators found that the derailment was caused by a switch malfunction triggered by a loose nut from track, and suspected that a repairman failed to tighten it during maintenance the previous night.

[166] The switch's detectors signalled a problem earlier, however, a second maintenance crew failed to find the loose nut and didn't properly communicate the fact to the control center, which then allowed the train on the track.

Among the various factors that were considered to be vectors of discomfort were the angle of joints and specific areas of pressure, which were discovered to be present after an analysis of questionnaires that were completed by recent passengers.